1. Field of the Invention
The present invention relates to a recording method, a recording and reproducing method, and a reproducing method for optical information for recording and reproducing the information on an optical recording medium by using a laser beam. The present invention also relates to an apparatus for carrying out the foregoing methods.
2. Description of the Related Art
The apparatus for recording and reproducing optical information is used such that an information mark is recorded on an information track of an optical recording medium by using a laser beam, and information is reproduced by detecting an optical change depending on the presence or absence of the information mark. Such an apparatus includes a focusing lens which is used to collect the laser beam onto the optical recording medium. The minimum diameter of a light spot formed on the optical recording medium is substantially represented by xcex/NA on the basis of the wavelength xcex of the laser beam and the numerical aperture NA of the focusing lens. In order to improve the recording density of the optical recording medium, it is necessary to decrease the arrangement interval (mark pitch) of information marks in the scanning direction of the light spot and the track interval (track pitch) for recording the information marks.
However, if the mark pitch and the track pitch are smaller than the light spot diameter, the following problem arises. That is, when one information mark is irradiated with the light spot, then a part of another information mark disposed therearound is simultaneously irradiated with the light spot, and a signal of the information mark to be reproduced is contaminated with any signal of the information mark disposed therearound. The contamination behaves as a noise component to cause interference, and the accuracy of reproduction is lowered. As described above, the contamination of the signal of the surrounding information mark greatly obstructs the realization of high density in the case of the system provided with the focusing lens and the laser having a specified wavelength.
The magnetic amplifying magneto-optical system (hereinafter referred to as xe2x80x9cMAMMOSxe2x80x9d) is known as a technique which makes it possible to solve the problem as described above (Japanese Patent Application Laid-Open No. 8-2350, WO98/02877, and WO98/02878). When MAMMOS is used, a minute magnetic domain, which is transferred from a recording layer to a reproducing layer, can be magnified by using a reproducing magnetic field to have a spot size of a reproducing light beam. Therefore, it is possible to remarkably increase the reproduced signal intensity. The magnified magnetic domain can be instantaneously erased after the reproduction. Therefore, the crosstalk is avoided. That is, in the case of MAMMOS, it is possible to greatly improve the reproduced signal intensity and S/N even when the mark pitch and the track pitch are decreased.
Japanese Patent Application Laid-Open No. 10-92036 describes a system which resides in a recording and reproducing method based on a principle different from that of MAMMOS. In this system, no magnetic field is applied during reproduction. The temperature gradient, which is generated by radiating a light beam for the magnetic domain included in a magnetic recording medium, is used to move the magnetic wall of a recording mark in a reproducing layer without changing the recording data in a recording layer so that the recording mark, which has a size of not more than a diffraction limit of the light beam, is reproduced. This reproducing method involves the following problem. That is, the movement timing is discrepant between the process of magnetic wall movement in the direction to magnify the recording mark and the process of magnetic wall movement in the direction to reduce the recording mark. As a result, the period of time corresponding to the recording mark is always detected as a period of time which is shorter by a certain period of time. In this method, in order to deal with this problem, the recording mark is formed (recorded) to have a length which is longer than ordinary one.
Japanese Patent Application Laid-Open Nos. 63-281229 and 4-265522 disclose a technique in which the edge position of the recording mark is corrected. The problem, i.e., the edge position of the mark is deviated from the position at which the recording is essentially performed, is solved by correcting the recording laser beam pulse in conformity with the mark length. However, in the case of the technique described in these patent documents, when a mark of nT is recorded, the mark length formed on the recording medium is still nT. There is no disclosure of the method of the present invention in which a recording mark shorter than nT is recorded for a recording signal corresponding n clocks.
As described above, MAMMOS is an extremely effective method to remarkably improve the recording density. However, according to an experiment performed by the present inventors, it has been revealed that when information is intended to be reproduced by using MAMMOS, the optimum values of the light power and the magnetic field intensity to be applied during reproduction of information differ depending on the mark length of a recorded mark. For this reason, it has been difficult in some cases to reliably reproduce pieces of data recorded as mark arrays including several types of marks having different lengths respectively. This problem will be explained below.
When the recording and the reproduction based on MAMMOS were evaluated, a laser having a wavelength xcex=680 nm and a focusing lens having an NA=0.55 were used, in which the NRZ modulation was applied with a channel bit length T=0.4 xcexcm. The channel bit length herein means the length on the recording medium corresponding to the unit bit of the modulated data array. In the following description, the recording mark length is varied to use three types of 1T, 2T, and 3T. If the mark length is longer than the spot diameter (=xcex/NA=1.24 xcexcm), the difference disappears concerning the characteristic when the reproduction is performed on the basis of MAMMOS. Therefore, explanation will be omitted for mark lengths of not less than 4T. Those having been known as the recording system for the magneto-optical recording medium include, for example, the optical modulation system, the magnetic field modulation system, and the optical magnetic field modulation system. In the experiment described below, the optical magnetic field modulation system was selected, in which the minute mark can be accurately formed.
As shown in FIG. 7A, the recording light power 203 was modulated at a constant cycle during the recording in accordance with the optical magnetic field modulation system, while the magnetic field of +Hw synchronized with the cycle was applied for a period of nT to record a mark having a length of nT (n is a natural number). For example, when it is intended to successively record a cycle of 1T mark and 1T gap, the magnetic field is an applied magnetic field 300 during recording at 1T cycle as shown in the drawing. When it is intended to successively record a cycle of 2T mark and 2T gap, the magnetic field to be applied during recording is an applied magnetic field 301 during recording at 2T cycle as shown in the drawing. When it is intended to successively record a cycle of 3T mark and 3T gap, the magnetic field to be applied during recording is an applied magnetic field 302 during recording at 3T cycle as shown in the drawing. The modulation of the light power and the waveform used when the magnetic field is applied are generated on the basis of a clock signal 113. The clock signal 113 is generated by a conventional synchronized signal-generating circuit (PLL) for outputting a signal synchronized with clock pits previously formed on the recording medium.
For example, 1 cycle of the clock signal 113 generated in this experiment corresponds to 0.1 xcexcm. Therefore, the following procedure is preferably adopted. That is, when a mark having a length of 1T(=0.4 xcexcm) is recorded, the applied magnetic field is +Hw for a period of four cycles of the clock signal. When it is intended to provide a gap having a length of 1T(=0.4 xcexcm), the applied magnetic field is xe2x88x92Hw for a period of four cycles of the clock signal. Similarly, when a mark having a length of 2T(=0.8 xcexcm) is recorded, the applied magnetic field is +Hw for a period of eight cycles of the clock signal. When a mark having a length of 3T(=1.2 xcexcm) is recorded, the applied magnetic field is +Hw for a period of twelve cycles of the clock signal. When it is intended to provide a gap having a length of 2T (=0.8 xcexcm), the applied magnetic field is Hw for a period of eight cycles of the clock signal. When it is intended to provide a gap having a length of 3T(=1.2 xcexcm), the applied magnetic field is xe2x88x92Hw for a period of twelve cycles of the clock signal.
During the reproduction, the reproducing light power 205 having a constant value Pr as shown in FIG. 6 is radiated onto the magnetic recording medium recorded with a group of recording marks 201. Further, the reproducing magnetic field 206 having a magnetic field intensity of xc2x1Hr synchronized with the clock signal 113 is applied. Thus, the reproduced signal 104 can be obtained. However, if the reproducing light power 205 is smaller than Pr which is the optimum value, or if the reproducing magnetic field 206 is smaller than Hr which is the optimum value, then any error arises in the reproduced signal. For example, erroneous reproduced signals 104-1 to 104-3 as shown in FIG. 6 are obtained. On the other hand, if the reproducing light power 205 is larger than Pr which is the optimum value, or if the reproducing magnetic field 206 is larger than Hr which is the optimum value, then any error arises in the reproduced signal in the same manner as described above. For example, erroneous reproduced signals 104-4 to 104-6 as shown in FIG. 6 are obtained.
FIG. 7B shows a result of investigation on the combination of the reproducing light power 205 and the reproducing magnetic field 206 capable of obtaining the reproduced signal involving no error as in the reproduced signal 104. When the cycle of 1T mark and 1T gap was successively recorded, Pr and Hr capable of obtaining the reproduced signal involving no error resided in the combinations indicated by xe2x80x9c1xe2x80x9din FIG. 7. Similarly, when the cycle of 2T mark and 2T gap was successively recorded, and when the cycle of 3T mark and 3T gap was successively recorded, then Pr and Hr capable of obtaining the reproduced signal involving no error resided in the combinations indicated by xe2x80x9c2xe2x80x9dand xe2x80x9c3xe2x80x9drespectively. According to this result, it is understood that Pr and Hr capable of obtaining the reproduced signal involving no error reside in the function of the mark length. The data of a user is recorded as a mark array composed of marks having several types of lengths. Therefore, it is necessary to select the combination of Pr and Hr so that the reproduced signal involving no error may be obtained stably for the marks having all of the lengths. However, as clarified by FIG. 7B, Pr and Hr, which are able to correspond to all of the mark lengths, are not present. In this situation as it is, a problem arises in that it is impossible to ensure the reliability during the reproduction.
Japanese Patent Application Laid-Open Nos. 63-281229, 4-265522, and 10-92036 neither disclose nor suggest the problem as described above and any means for solving the problem.
The present invention has been made in order to solve the problem as described above, an object of which is provide a recording method, a reproducing method, and a reproducing apparatus for an optical recording medium, which make it possible to always reproduce data in a stable manner irrelevant to the length of a recording mark represented by a number of clocks (integral multiple of nT).
According to a first aspect of the present invention, there is provided a recording method on an optical recording medium for recording, as information, recording marks having a variety of lengths on the basis of a clock, on the optical recording medium including at least a recording layer, the method comprising forming a recording mark having a length shorter than nT (T is a channel bit length)on the optical recording medium as a recording signal corresponding to an amount of n clocks (n is an integer). The recording signal, which corresponds to the amount of n clocks (n is an integer), is converted into a recording signal shorter than the original recording signal. The recording mark, which has the length shorter than nT, can be formed on the optical recording medium on the basis of the recording signal after being converted.
The principle of the present invention will be explained with reference to FIG. 5, as compared with the conventional recording method. A magneto-optical recording medium is used as the recording medium. The optical magnetic field modulation system is adopted, in which a magnetic field modulated on the basis of the recording signal is applied, while radiating a pulse beam having a constant cycle is radiated onto the magneto-optical recording medium. The NRZ modulation is used for the modulation system for the recording code.
As shown in FIG. 5, when the mark having the length of nT is recorded in accordance with the conventional system, then the recording magnetic field 211 is +Hw for a period of time of nT, and the recording magnetic field is xe2x88x92Hw in regions in which the recording mark is not recorded. However, as described above, the reproducing power Pr and the applied magnetic field Hr, which are capable of corresponding to all of the mark lengths, are not present in the case of the conventional recording system. Therefore, it is impossible to ensure the reliability during the reproduction. On the other hand, in the case of the recording system according to the present invention, the mark length correction amount Lc is provided for each mark length as shown in FIG. 5 so that the mark length is shortened. For example, it is assumed that the correction amount for the 1T mark is Lc1, the correction amount for the 2T mark is Lc2, and the correction amount for the 3T mark is Lc3. On this assumption, when the 1T mark is recorded in accordance with the system of the present invention, then the recording magnetic field 204 is +Hw for a period of time of 1Txe2x80x2(=1T+2Lc1), and the recording magnetic field is xe2x88x92Hw in the regions in which the mark is not recorded. That is, the 1T mark is recorded not as a mark having a length of 1T but as a mark having a length of 1Txe2x80x2. The broken line concerning the recording magnetic field 204 indicates the conventional recording waveform. Similarly, the 2T mark and the 3T mark to be recorded are recorded as marks having lengths of 2Txe2x80x2 and 3Txe2x80x2 respectively, in which there are given 2Txe2x80x2=2T+2Lc2 and 3Txe2x80x2=3T+2Lc3. Marks having lengths of not less than 4T are subjected to the correction for the mark length exactly in the same manner as described above. Whether the mark length correction amount Lcn corresponding to a mark having a length of nT has a positive, zero, or negative value changes depending on, for example, the characteristic and the structure of the medium, the reproducing power, and the applied magnetic field. However, the mark length correction amount Lcn has a value which is not zero for at least one n.
Explanation will be made with reference to FIGS. 8A and 8B for the effect obtained by recording the mark having each mark length nT as the mark having the mark length nTxe2x80x2 corrected by using the mark length correction amount Lc prescribed for each mark length when information is recorded in accordance with the present invention. FIG. 8A shows the same recording and reproducing conditions as those shown in FIG. 7A except that an identical mark length correction amount Lc=xe2x88x920.1 xcexcm was added to each mark length. FIG. 8B shows, for each mark length, the relationship between the reproducing power Pr and the applied magnetic field Hr to obtain the good reproduced signal under the conditions shown in FIG. 8A. However, the intensity Pw of the recording light power 203 and the intensity Hw of the recording magnetic field after correction are designed so as to obtain the largest number of combinations of the reproducing light power 205 and the reproducing magnetic field 206 with which the reproduction can be performed without any error to reproduce the cycle pattern of the 1T mark and the 1T gap recorded by using the conventional recording magnetic field 211 (see FIG. 5). Since there is given Lc=xe2x88x920.1 xcexcm, the substantial mark length of 1T is 0.2 xcexcm, the substantial mark length of 2T is 0.6 xcexcm, and the substantial mark length of 3T is 1 xcexcm. When FIG. 7B is compared with FIG. 8B, it is understood concerning each mark length that the combination of Pr and Hr, with which the reproduction can be performed without any error, is increased when Lc=xe2x88x920.1 xcexcm is added, probably because of the following reason. That is, when the mark length of the recording mark is shortened, the leak magnetic field, which is brought about by another recording mark which adjoins with the gap intervening therebetween, is decreased. As a result, the error of the reproduced signal from the recording mark to be reproduced is reduced. However, the combination of Pr and Hr capable of corresponding to all of the mark lengths is not present, in the same manner as in the case in which Lc is not added.
FIGS. 9A and 9B show recording and reproducing conditions and results of measurement obtained when an identical mark length correction amount Lc=xe2x88x920.2 xcexcm was added to each mark length. Pw and Hw are the same as those shown in FIG. 8A. Since there is given Lc=xe2x88x920.2 xcexcm, the substantial mark length of 1T is 0.1 xcexcm, the substantial mark length of 2T is 0.5 xcexcm, and the substantial mark length of 3T is 0.9 xcexcm. Concerning the mark lengths of 2T and 3T, the combination of Pr and Hr, with which the reproduced signal involving no error is obtained, is increased when Lc=xe2x88x920.2 xcexcm is added. However, concerning the mark length of 1T, the combination of Pr and Hr capable of performing reproduction without any error is not present.
According to the results described above, the following fact is comprehensible. That is, concerning the mark of the length of not less than 2T, the combination of Pr and Hr, with which the reproduced signal involving no error is obtained, is increased in the case of Lc=xe2x88x920.1 xcexcm. As for the mark of the length of 1T, the combination of Pr and Hr, with which the reproduced signal involving no error is obtained, is increased in the case of Lc=xe2x88x920.2 xcexcm. Therefore, it is preferable that the mark length correction amount Lc is changed depending on the length of the mark when the mark is subjected to the recording. FIGS. 10A and 10B show recording and reproducing conditions in which Lc=xe2x88x920.1 xcexcm was added when the mark of the length of 1T was recorded, and Lc=xe2x88x920.2 xcexcm was added when the mark of the length of not less than 2T was recorded. FIGS. 10A and 10B also show results of measurement, for each mark length, of the relationship between the reproducing power Pr and the applied magnetic field Hr with which the reproduced signal involving no error is obtained. Although the combination of Pr and Hr capable of corresponding to all of the mark lengths is not present at all in the results shown in FIGS. 7B, 8B, and 9B, it is understood that a plurality of such combinations are present in the results shown in FIG. 10B. The reason of the foregoing results is considered as follows. That is, the leakage occurs from another recording mark which adjoins the recording mark to be reproduced, with the gap intervening therebetween. The longer the mark length of the another recording mark is, the larger the leak magnetic field is. Therefore, when the recording is performed by increasing the mark length correction amount Lc in accordance with the increase in the length of the recording mark, it is possible to decrease the leak magnetic field which is exerted on the concerning recording mark by the another recording mark adjacent to the concerning recording mark to be recorded. Accordingly, it is possible to reduce the error of the reproduced signal obtained from the concerning recording mark.
As described above, in the case of the use of the information-recording and reproducing system according to the present invention in which the mark length correction amount Lc is changed depending on the length of the mark when the mark is subjected to the recording, it is possible to select the combination of Pr and Hr with which the reproduced signal involving no error is obtained for the marks of all of the lengths. As a result, it is possible to sufficiently ensure the reliability during the reproduction.
In the method of the present invention, the reproduction can be performed for the marks of all of the lengths under an identical reproducing condition provided that there is at least one combination of Pr and Hr with which the reproduced signal involving no error is obtained for the marks of all of the lengths. Further, the margin for the reproducing condition is widened when appropriate Lc is selected depending on n so that the number of combinations is maximized.
The foregoing results are obtained under the condition in which the recording parameters Pw and Hw have been already determined. However, when Pw and Hw are not optimized, the following procedure is available. That is, optimum Lc""s are determined for the respective mark lengths in the same manner as described above respectively, while successively changing Pw and Hw at certain prescribed intervals starting from a preset initial value to adopt Pw and Hw with which the number of combinations of Pr and Hr capable of obtaining the reproduced signal involving no error is maximized.
According to the recording method of the present invention, the margin is widened concerning the reproducing magnetic field intensity and the reproducing light beam intensity to reliable perform the reproduction for a variety of mark lengths. Therefore, it is effective to apply the present invention to the optical recording medium of the type in which the light and the magnetic field are applied during the reproduction, for example, the magneto-optical recording medium. Especially, it is more effective to apply the present invention to a magneto-optical recording medium comprising a magnetic recording layer and a reproducing layer, in which a recording mark recorded in the magnetic recording layer is transferred in a magnified form to the reproducing layer by radiating a light spot and applying a magnetic field during reproduction. The present invention is also effective with respect to the magneto-optical recording medium to perform reproduction based on the magnetically induced super resolution (MSR).
According to a second aspect of the present invention, there is provided a reproducing method for reproducing information recorded on an optical recording medium including recording marks having a variety of lengths formed thereon on the basis of a clock, the method comprising using the optical recording medium on which a recording mark having a length shorter than nT is formed (T is a channel bit length) as a recording signal corresponding to an amount of n clocks (n is an integer), and reproducing the information in synchronization with the clock while radiating a reproducing light beam.
In the reproducing method described above, it is possible to perform the reproduction with a wide reproducing margin in a reliable manner on the optical recording medium, preferably on a magneto-optical recording medium subjected to the recording with a variety of mark lengths in accordance with the first aspect. The reproducing method is especially preferred when the optical recording medium is a magneto-optical recording medium comprising a magnetic recording layer and a reproducing layer, in which a recording mark recorded in the magnetic recording layer is transferred in a magnified form to the reproducing layer by radiating a light spot and applying a magnetic field during the reproduction. In this case, it is possible to ensure the sufficient power margin for the reproducing light beam and the reproducing magnetic field, and it is possible to reproduce the recording marks having the variety of lengths at high C/N.
According to a third aspect of the present invention, there is provided a recording apparatus for an optical recording medium, comprising:
a clock-generating circuit;
a recording length-correcting circuit for correcting a recording signal length so that a recording signal corresponding to an amount of n clocks (n is an integer) is formed as a recording mark having a length shorter than nT (T is a channel bit length) on the optical recording medium; and
an applying unit for modulating and applying at least one of a light beam and a magnetic field to the optical recording medium on the basis of an output from the recording length-correcting circuit.
The recording length-correcting circuit of the recording apparatus of the present invention corrects the mark length nT with a mark length correction amount Lc. The recording length-correcting circuit may include a mark length-correcting circuit for converting a code length corresponding to a mark length into a code length shorter than the above. The mark length-correcting circuit may include a code conversion table to shorten the mark length.
The mark length-correcting circuit may be operated such that the shortness of the recording mark for nT is allowed to differ depending on n. Accordingly, any mark having different n can be subjected to the reproduction without changing the reproducing condition (for example, the reproducing light beam intensity and the reproducing magnetic field intensity). The recording length-correcting circuit may further includes an address retrieval memory, a mark length-identifying circuit, and a memory for storing a mark length obtained by being corrected by the mark length-correcting circuit.
The recording apparatus may further comprise a data modulator, a data demodulator, and a detector for detecting a reflected light beam from the optical recording medium, thereby making it possible to reproduce the information. In other words, the recording apparatus of the present invention may function as a recording and reproducing apparatus. In this arrangement, a magnetic field-applying unit may be used to apply a reproducing magnetic field to the optical recording medium when the information is reproduced. The data demodulator may be used so that the recording mark having the length shorter than nT recorded on the optical recording medium is demodulated to produce the signal corresponding to the amount of n clocks.